1. Field of the Invention
The present invention relates to an anti-lock brake controlling apparatus, and more particularly to an anti-lock brake controlling apparatus in which a braking force is controlled on the basis of a vibration characteristic appearing in the rotational speed of a wheel (wheel speed), so as to effect a braking operation such that the coefficient of friction between the wheel and the road surface reaches a maximum value (a peak value of .mu.).
2. Description of the Related Art
A conventional anti-lock brake controlling apparatus prepares a vehicle-speed signal, a vehicle-acceleration/deceleration signal, or a speed signal approximating the vehicle speed on the basis of a signal from a wheel speed sensor, and the braking force is controlled by comparing them so as to effect an anti-lock braking operation.
That is, Japanese Patent Application Laid-Open No. 61-196853 discloses an anti-lock brake controlling apparatus in which the possibility of wheel locking is determined from a comparison between an estimated vehicle speed and a reference speed obtained from the wheel speed or the like which are estimated, and the braking force is reduced when there is a possibility of wheel locking. In this anti-lock brake controlling apparatus, an estimated vehicle speed v.sub.v is obtained by connecting the troughs of a speed v.sub.w, obtained from the wheel speed, by a fixed gradient, as shown in FIG. 1. However, it can be seen that there is a difference between the estimated vehicle speed v.sub.v and an actual vehicle speed v.sub.v*.
In addition, in this anti-lock brake controlling apparatus, to prevent the estimated vehicle speed v.sub.v from becoming greater than the actual vehicle speed v.sub.v* due to a change in the wheel ground-contact load during traveling on a bad road, the rate of increase in the estimated vehicle speed is suppressed in a case where the changes in wheel speed are more than those in the estimated vehicle speed.
If brakes are applied when a vehicle is traveling at a certain speed, slip occurs between the wheels and the road surface, and it is known that, as shown in FIG. 2, the coefficient of friction .mu. between the wheel and the road surface changes with respect to a slip ratio S which is expressed by the following Formula (1). EQU S=(v.sub.v* -v.sub.w)/v.sub.v* ( 1)
where v.sub.v* is the actual vehicle speed, and v.sub.w is the wheel speed.
In this .mu.-S characteristic, the coefficient of friction .mu. reaches a peak value at a certain slip ratio (in a region A2 in FIG. 2). If the slip ratio at which the coefficient of friction .mu. reaches the peak value is known in advance, the slip ratio can be controlled by determining the slip ratio from the vehicle speed and the wheel speed.
For this reason, with the anti-lock brake controlling apparatus disclosed in Japanese Patent Application Laid-Open No. 1-249559, the slip ratio is calculated from an approximate value of the vehicle speed and the wheel speed or the like, and the braking force is controlled on the basis of a comparison between the calculated slip ratio and a set slip ratio. With this anti-lock brake controlling apparatus, a measure is provided not to set the brake pressure in a state of lowered pressure for a period more than necessary in order to prevent the vehicle from being set in a state of no brake for a long time due to the difference between the estimated vehicle speed v.sub.v and the actual vehicle speed v.sub.v*.
As shown in FIG. 3, these conventional anti-lock brake controlling apparatuses are each comprised of: a vehicle-speed estimating section 2 for estimating the estimated vehicle speed v.sub.v from a wheel speed .omega..sub.w and a vehicle acceleration v.sub.v ' (=dv.sub.v /dt); and a braking-force controlling section 3 for controlling a braking force P.sub.b with respect to a driving system 1 of the vehicle by detecting the locked state of the wheel from the wheel speed .omega..sub.w and the estimated vehicle speed v.sub.v.
However, with such conventional anti-lock brake controlling apparatuses, since the vehicle-speed estimating section is required, it is necessary to return the braking force until the speed v.sub.w, determined from the wheel speed, and the actual vehicle speed v.sub.v* agree with each other or become close values, as shown in FIG. 1. For that reason, the increase and decrease of the braking force applied to the wheels need to be repeated at a relatively low frequency. In addition, since the estimated vehicle speed, as compared with a reference speed, is an approximate value determined from the wheel speed and the vehicle acceleration or the like, the estimated vehicle speed sometimes differs substantially from an actual vehicle speed. Hence, there have been problems in that, in some cases, the wheels lapse into a locked state for a long period of time, and that the braking force is reduced extremely for the purpose of returning to an unlocked state. Therefore, a considerable effect is exerted on the behavior of the vehicle, possibly resulting in an increase in the braking distance and uncomfortable vibrations.
Further, with the anti-lock brake controlling apparatus for controlling the braking force on the basis of the slip ratio, it can be readily estimated that the slip ratio at which the coefficient of friction becomes maximum varies depending on the state of the road surface on which the vehicle travels. As a measure against this problem, it has conventionally been necessary to detect and estimate states of the road surface, and prepare a plurality of reference slip ratios corresponding to the states of the road surface, or change the reference slip ratio in correspondence with the state of the road surface.